Basic reproduction number

R0 (pronounced 'R-naught' or 'R-zero') is the basic reproduction number — the average number of new infections one infected person would produce if introduced into a fully susceptible population with no interventions in place. R0 > 1 means an epidemic can grow; R0 < 1 means the chain of transmission burns out. R0 is a property of the pathogen interacting with a specific host population at a specific moment, not a fixed constant.

Reported R0 values vary widely across pathogens: measles is among the highest at 12 to 18, pertussis at 12 to 17, varicella at 8 to 10. COVID-19's ancestral strain had an R0 around 2 to 3; the BA.1 Omicron variant climbed to 8 to 10. Ebola is comparatively low (1.5 to 2.5) — high case-fatality but limited person-to-person spread compared with respiratory pathogens. Most hantaviruses have R0 below 1 in humans because rodent-to-human transmission dominates and human-to-human spread (except Andes virus) is not documented.

R0 is often confused with R-effective (R-eff or R-t), which is the reproduction number in the actual current population with whatever immunity and behavioral changes are in place. R-eff is what public-health responses target — pushing R-eff below 1 ends outbreaks. R0 is the theoretical baseline; R-eff is the live measurement.